The growing exploration of oil wells has increased the demand for plug and abandonment (P&A) operations. These mandatory operations are dedicated to creating a series of permanent barriers that effectively prevent potential fluid leakages, protecting the environment from contamination. However, traditional techniques employed for well sealing rely on the cementing process, which can be time-consuming and costly. In the search for cost-effective alternatives, an innovative thermal proposal for plugging wells, termed Thermal Plug and Abandonment (TP&A), is numerically investigated in this work. This new alternative proposes inserting an exothermic chemical reaction, such as thermite, through the casing or production tubing until reaches the desired depths. The reaction generates enough heat to melt the in-situ surrounding materials, and after cooling, the solidified mass composed of these materials plus the thermite products forms the well seal. Alternatively, the heat generated by the reaction can be utilized to melt only the production tubing, creating an intentional pathway for inserting the cement and thereby reducing operational costs significantly. To investigate this procedure, a numerical model with chemical kinetics, phase change, and multilayered heat diffusion is developed using the finite volume method through the OpenFOAM® toolkit. Initially, this work focuses on validating the kinetic model by simulating small-scale experiments. The results from the first validation analyses indicated that the surrounding material could affect the reaction ignition, heat generation, and temperature levels, particularly next to the interface. In addition, the second validation analysis indicates that the heat transfer within the thermite region can be enhanced with a denser mixture and slower reaction propagation. Once validated, the chemical model is incorporated into simulations with the TP&A scenario to investigate the heat diffusion and phase change through the oil well structure. In this context, considering the dilution of the initial thermite mixture with a specific quantity of inert alumina appears as a promising alternative to enhance heat transfer within the system. This approach might improve the melting of the production tubing, thereby promoting the alternative TP&A procedure, which focuses on the tubing's melting to establish a pathway for the insertion of cement.